Recent years have seen significant progress in the development of engineered ceramics. These new materials are expected to find uses in structural applications at high temperatures or in other specific areas where metals or polymers are less desirable. Because of the combination of attractive mechanical, thermal, and thermo-mechanical properties, one of the most interesting ceramics is Si.sub.3 N.sub.4. Silicon nitride can exist in alpha or beta hexagonal structures, the properties of the two differing somewhat. Beta-Si.sub.3 N.sub.4 (.beta.-Si.sub.3 N.sub.4) is preferred because it generally exhibits higher strength, modulus, and hardness. Si.sub.3 N.sub.4 has high strength at high temperatures, good thermal stress resistance due to its low coefficient of thermal expansion, and high resistance to oxidation when compared to other non-oxides. These properties thus allow Si.sub.3 N.sub.4 components to be used to higher operating temperatures.
Potential applications for Si.sub.3 N.sub.4 materials are abundant, including use in combustion system components such as high temperature turbines, combustion chamber liners, rocket nozzles, and thrust deflectors. The low thermal conductivity of Si.sub.3 N.sub.4 makes it attractive as a thermal barrier, either as a coating or stand-alone component. Its low density allows it to replace superalloys with a 40% weight savings. Silicon nitride's dielectric properties are of interest for use in low observable (stealth) technology.
The conventional method for producing Si.sub.3 N.sub.4 is via the hot-pressing of powder. Generally, this requires the addition of a sintering aid which lowers the melting temperature at the grain boundaries to permit consolidation. Thus the ultimate utilization temperature of the component is reduced to well below that of pure Si.sub.3 N.sub.4, preventing full use of the material's advantages. Hot pressing of Si.sub.3 N.sub.4 with sintering aids such as mixtures of yttria and alumina is typically conducted at temperatures ranging from 1600.degree. C. to 2000.degree. C., usually above 1700.degree. C.
A technique used to produce pure Si.sub.3 N.sub.4, usually as a coating, is chemical vapor deposition (CVD). In CVD gaseous reactants are caused to flow over a heated substrate where they react, depositing one or more phases, and often producing gaseous byproducts. For Si.sub.3 N.sub.4 the reactants are typically silanes, chlorosilanes, or chlorides reacted with ammonia.
In the production of Si.sub.3 N.sub.4 there are distinct advantages to the use of CVD. Crystalline CVD Si.sub.3 N.sub.4 has demonstrated one-tenth the oxidation rate of hot-pressed material. The density of CVD material is easily near theoretical. CVD also has high throwing power (can uniformly coat surfaces not in the line of sight of the source). The CVD processing temperature for many materials is much lower than that for other production methods. Although CVD is generally used for coatings, it can and has been used to prepare stand-alone bodies of pure material by depositing on a mandrel which is then removed.
A difficulty in the CVD of Si.sub.3 N.sub.4 is that only amorphous material is deposited below about 1200.degree. C., and near 1200.degree. C. crystalline material is deposited at only very low deposition rates (5 um/h). Increasing the deposition rate via increased reactant flow quickly causes the deposit to be amorphous. The only exception is the deposition from highly dilute silane and ammonia, which deposits polycrystalline Si.sub.3 N.sub.4 at 1100.degree. C. at relatively low rates (&lt;10 um/h). Attempts to crystallize deposited amorphous coatings at 1500.degree. C. results in the coating disintegrating to a fine powder.
Unfortunately, amorphous Si.sub.3 N.sub.4 is not desirable for most engineering applications. Deposited amorphous coatings greater than 1 um in thickness are heavily microcracked. Amorphous material also tends to retain contaminants that result from processing (e.g., HCl).
Another technique which utilizes CVD to prepare ceramic bodies is chemical vapor infiltration (CVI). In CVI the vapor phase reactants are caused to diffuse or flow through a porous perform where they deposit material on the contacted surfaces, filling the void space and forming a composite material. The preform is typically fibrous, allowing a fiber-reinforced composite to be produced at relatively low temperatures and without causing damaging stress to the fibers. This technique has been successfully used to produce amorphous Si.sub.3 N.sub.4 matrix-fiber reinforced material. Again, because of the amorphous nature of the matrix, the composite is of low strength (about one-half that of similar SiC-matrix material), and retains significant amounts of HCl, which causes chemical instability.
Raising the CVI processing temperature to at least 1300.degree. C. could form crystalline Si.sub.3 N.sub.4. However, this is not feasible for CVI. At such high temperatures the rate of deposition, which is governed by the exponential Arrhenius relation, is too high for effective infiltration to occur. Deposition tends to occur at the outer surface, eventually sealing the porosity thereof and creating a steep density gradient between the entrance surface and the center of the preform. In addition, many of the types of ceramic fibers used for CVI processing suffer substantial degradation at temperatures much in excess of 1100.degree. C.,
The deposition temperature for crystalline Si.sub.3 N.sub.4 has been reduced, and/or the physical, chemical, and mechanical properties of the deposit have been altered by the addition of dopants or contaminants. One approach was the addition of titanium in the form of titanium nitride. Titanium tetrachloride was added to a SiCl.sub.4 --NH.sub.3 --H.sub.2 reaction to produce a crystalline Si.sub.3 N.sub.4 coating with a dispersed TiN second phase. The deposition temperatures for the crystalline material were at least 1250.degree. C. At 1250.degree. C., alpha-type Si.sub.3 N.sub.4 was deposited, and at temperatures greater than 1400.degree. C., .beta.-Si.sub.3 N.sub.4 was produced. The materials were developed for improved thermal and electrical conductivity. TiN, however, degrades the corrosion/oxidation resistance properties of the Si.sub.3 N.sub.4 base material. Also, the deposition temperature for the preferred .beta.-Si.sub.3 N.sub.4 is still above the decomposition temperature of many otherwise suitable substrates such as silicon, nickel, nickel-based alloys, and many ceramic fibers. Examples of ceramic fibers are: Nicalon, a trade name for a Si--C--O manufactured by Nippon Carbon Co., Tokyo, Japan; Nextel, a trade name for Al--Si--B--O manufactured by 3M Corp., St. Paul, Minn.; and HPZ, a trade name for Si--N--C--O manufactured by Dow-Corning, Midland, Mich.
There is a need for a second phase material which improves chemical resistance, lowers the deposition temperature for crystalline material, especially that which contains .beta.-Si.sub.3 N.sub.4, and also provides a Si.sub.3 N.sub.4 coating with exceptional oxidation/corrosion resistance and chemical inertness.
For further information, the following documents are referenced, and the disclosure of each is expressly incorporated herein by reference:
1. R. A. Tanzilli, et al., "Processing Research on Chemically Vapor Deposited Silicon Nitride--Phase 3", Document No. 81SDR2111, final technical report prepared under contract No. N0014-78-C-0107 for the Office of Naval Research, (1981).
2. T. Hirai, "CVD of Si.sub.3 N.sub.4 and Its Composites, " pp. 329-345 in Emergent Process Methods for High Technology Ceramics, North Carolina State University, Ed. R. F. Davis, H. Palmour III, and R. L. Porter, Plenum Press, N.Y. (1984).
3. T. Hirai, et al., "CVD Fabrication of In-situ Composites of Non-oxide Ceramics," Tailoring Multiphase and Composite Ceramics, Proceedings of the Twenty-first University Conference on Ceramic Science, p. 165-178, Pennsylvania State University, Plenum Press, New York (1986).
4. F. Galasso, et al., "Pyrolytic Si.sub.3 N.sub.4," J. Am. Ceram. Soc. 55(8), 431 (1972).
5. J. J. Gebhardt, et al., "Chemical Vapor Deposition of Silicon Nitride," J. Electrochem. Soc.: Solid-State Science and Technology 123(10), 1578-1582 (1976).
6. A. C. Airey, et al., "Pyrolytic Silicon Nitride Coatings," Proc. Brit. Ceram Soc. 22, 305-320 (1972).
7. U.S. Pat. No. 4,598,024.
8. U.S. Pat. No. 4,580,524.